\"additive\" or \"subtractive\" pressuretrol?

Larry Peck

I own an older home with one-pipe steam heat. I understand from Dan's great book, "The Lost Art of Steam Heating," that the pressuretrol cut-in should be 1/2 psi and the cut-out should be 1 1/2. However, I do not know whether the pressuretrol is "additive" or "subtractive."

The pressuretrol is a White Rodgers, Type 1204-5, Style B-2. I do not know the age, but it is not new. Removing the metal cover plate reveals two wheels, a large lower wheeel numbered from 0 to 10 in 1/2 psi increments but having no other markings, and a small upper wheel marked "PSI Diff." My question is whether the large lower wheel is the cut-in pressure (indicating an "additive" pressuretrol) or the cut-out pressure (indicating a "subtractive" pressuretrol).

Also, how do I know if the pressuretrol even works if the pressure gauage no longer functions?

Many thanks in advance for you replies.

David Efflandt

get working pressure gauge

White Rodgers has nothing about pressure controls on their website. But since the one dial goes down to zero, I would assume that would be cut-in and differential would be additive.

You should really get a working pressure gauge, but it is hard to find one high enough for relief setting that indicates below 2 psi, so I use a pressure/vacuum gauge. That way I can see it rise from zero and shut off before 2 psi (minimum settings on my Honeywell pressuretrol are 0.5 psi cut-in and 1 psi rise). Original gauge when I moved into home had plugged connection, so it was just reading trapped pressure instead of boiler pressure, and would not even go all the way down when removed (stuck).

[Deleted User]

The WR is

additive. The MH w/ the plastic cover (L Series) is subtractive. The MH w/ the metal cover (PA Series) is additive. That is, when it works.

Larry Peck

recommendation for new pressuretrol or vaporstat

Thank you very much for your prompt and informative replies.

Could anyone recommend a replacement pressuretrol? Should I purchase a vaporstate instead and, if so, could someone recommend a make and model number (which sits on top of the low water cutoff valve)? Does a pressuretrol make sense for an old boiler? Thanks in advance for any replies.

Larry